The present invention relates to methods and systems for controlling contamination from microorganisms in agricultural irrigation systems.
The addition of chlorine or chlorine compounds to water has been used for over a century to purify water. Chlorine purifies water by killing or inactivating microorganisms in the water. There are numerous types of microorganisms. The most common classes of microorganisms found in typical water sources utilized for agricultural irrigation are bacteria, viruses, and protozoa, which include coliforms and biofilms. Coliform is a group of microorganisms or bacteria whose presence in water is a common indicator of fecal pollution and potentially dangerous disease-causing (pathogenic) microorganisms. Coliform bateria include Escherichia coli (E. coli) which has been the culprit of multiple prior and current food recalls and outbreaks. Microbial biofilms are populations of organisms, typically surrounded by an extracellular, polymeric slime layer, which accumulate on surfaces in an aqueous environment. Biofilms are composed of microorganisms such as bacteria, protozoa, algae and invertebrate animals.
The mode of interaction between the microorganism and the chlorine normally is different for each class. Chlorine disinfects water, eliminating microorganism contamination, by several mechanisms. It destroys bacteria by penetration of the cell wall, killing the organism. It disinfects viruses by inactivation, rendering the organism incapable of reproduction. It disinfects giardia (a genus of protozoa which infect the gastrointestinal tract of humans when ingested) by inactivation. In other words, in the case of bacteria, the chlorine penetrates the cell wall and kills the organism. For viruses and protozoa, chlorine inactivates the organisms by rendering them incapable of reproduction.
Recommended uses of chlorine in agricultural water treatment include the removal of bacteria and algae (disinfection), the prevention of regrowth of algae and bacteria in the agricultural water distribution system (disinfection) and the oxidation of iron and manganese in tanks or reservoirs prior to filtration. Interference with the disinfection chlorination process is spawned by turbidity, the presence of excess chlorine-demanding inorganic compounds (e.g. iron, manganese) and high pH.
The primary sources of chlorine are chlorine gas, calcium hypochlorite and sodium hypochlorite. Chlorine gas is very effective (100% available chlorine), inexpensive and very dangerous. Chlorine gas was the first gas used in chemical warfare. Due to safety concerns, the use of chlorine gas for agricultural chlorination is prohibited in many regions of California and presumably elsewhere. Sodium hypochlorite is available as an easy to use liquid which has a low percentage of chlorine (6 to 12.5% available chlorine). Excess sodium can, however, be detrimental to plants, and sodium hypochlorite solution is corrosive and its high water content leads to high freight costs. Calcium hypochlorite is available as a solid which has a high level of chlorine (67% available chlorine). As a solid, calcium hypochlorite is very difficult to use in a pressurized system and the presence of calcium can cause plugging in drip irrigation systems using alkaline water.
Although chlorine and its related compounds are routinely used for controlling microorganisms in water systems, there has been no accurate and economical method for controlling chlorine in systems that are used for irrigation of agricultural crops. The farmer or grower conventionally attempts microorganism control by adding a “slug feed” of bleach or copper sulfate (a non-oxidizing algaecide/biocide) to the water of an irrigation system with little or no knowledge of the resultant additive concentration in the system or the abundance of the microorganism to be killed or controlled. Although the goal of controlling the microbiological organisms in the irrigation water system is correct, the slug-feed approach is normally inadequate. Excess chlorination is undesirable because excess available chlorine in the irrigation water can adversely affect (kill) probiotics which are naturally-occurring in, or are added by the grower to, the soil. Additionally, minimizing chlorination prevents build-up of chloride concentrations in the soil. These chlorides can be toxic to sensitive crops, and can result in undesirable decreases in crop yield or quality.
The deficiencies in the control of microorganisms in agricultural production have spawned repeated, and now well-publicized, out-breaks of food contaminations and recalls in the U.S. Crops such as spinach and lettuce have been determined to have been contaminated by E. coli during in-field production (rather than post-harvest handling). Such E. coli contaminations have caused severe illness and even death. One of the prime suspects of in-field crop contamination is contaminated irrigation water, which highlights a serious need to improve the conventional approaches to microorganism control in irrigation water
It is an object of the invention to provide a method and a system for accurately and economically delivering chlorine, or compounds that liberate chlorine, into an irrigation system for controlling the growth of microorganisms and for disinfecting the irrigation water.